A vessel such as a tanker usually stores water called ballast water in a ballast tank disposed on the vessel in order to balance the vessel under way while traveling toward another destination after unloading freight such as crude petroleum. Ballast water is normally charged at an unloading port and discharged at a loading port. Therefore, if these ports are located at different places, microorganisms such as plankton and bacteria in the ballast water comes to travel all over the world. Thus, ballast water is discharged at a loading port in a sea area different from that of an unloading port, and microorganisms in another sea area are released at the loading port and may destroy the ecosystem in the sea area. In order to prevent the marine environment from being destroyed by such ballast water, the International Maritime Organization (IMO) has entered into the International Convention for the Control and Management of Ships' Ballast Water and Sediments and has set a ballast water discharge standard to restrict the microorganism content in ballast water discharged from ships.
This ballast water discharge standard sets the discharge limits for plankton according to sizes, specifically, less than 10 individuals/m3 for plankton of 50 μm or more and less than 10 individuals/ml for plankton of 10 to 50 μm. The ballast water discharge standard also sets the discharge limits for bacteria, such as less than 250 cfu/100 ml for coliform bacteria.
Ballast water to be stored in a ballast tank is thus required to be detoxified by killing microorganisms in ballast water. As a measure for detoxifying ballast water by killing microorganisms therein, there are known treatment techniques of the filtration of ballast water by a ballast water treatment device including a filter that is disposed in a casing and filters ballast water and the ultraviolet irradiation to ballast water by an ultraviolet irradiation device. The filter of the ballast water treatment device used in this treatment technique is required to remove 99.99% of plankton having a size of 50 μm or more. Accordingly, a filtration body such as through a wire mesh with minimum apertures is needed. For this reason, clogging is frequently caused, and constant rinsing of a filter is considered to be important.
As a known device for removing foreign substances deposited on the inner surface of the filter, Patent Literature 1 discloses a filtration device including: a suction nozzle that opens at a position facing the inner surface of the filter; a nozzle moving unit that moves the suction nozzle along the inner surface of the filter in the both axial and circumferential directions of the filter; a backwash nozzle that is disposed in a position facing the suction nozzle outside the filter and emits backwash water; and a backwash nozzle moving unit that moves the back wash nozzle in synchronization with and in the same direction as the suction nozzle.
Rinsing of a filter by the filtration device according to Patent Literature 1 is performed by opening a sludge removal valve when the pressure difference between the inside and outside of the filter reaches a predetermined pressure or more, emitting backwash water from the backwash nozzle, removing suspended matter deposited on the filter with rinsing water, and discharging sludge from the suction nozzle, while the suction nozzle and the backwash nozzle are moved by the nozzle moving unit and the backwash nozzle moving unit.